backend-drm: Enforce content protection for hardware planes

62626cbfec ensures that the GL render will not render a view's content
to the screen when the surface has requested a higher content-protection
level than the output currently offers.

When the HDCP MR was split into the core content-protection support in !83
and specific DRM support for HDCP in !48 (not yet landed), this opened a
hole where the DRM backend could promote a view to a hardware plane,
even if the output offered a lower protection level than the surface
wanted to enforce.

In the DRM backend, check the desired protection level, and refuse to
promote the view to a hardware plane if the output does not offer
sufficient protection. This will lead to presentation falling back to
the renderer, which may censor the content, reduce quality, etc.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Fixes: 4b6e73d617 ("libweston: Add support to set content-protection for a weston_surface")

libweston/backend-drm/fb.c

@@ -497,6 +497,10 @@ drm_fb_get_from_view(struct drm_output_state *state, struct weston_view *ev)
 	if (!drm_view_transform_supported(ev, &output->base))
 		return NULL;
 
+	if (ev->surface->protection_mode == WESTON_SURFACE_PROTECTION_MODE_ENFORCED &&
+	    ev->surface->desired_protection > output->base.current_protection)
+		return NULL;
+
 	if (!buffer)
 		return NULL;